1. Field of the Invention
In a first aspect thereof, the present invention relates to a method for cutting a ribbon of rubber-coated fabric including a plurality of reinforcing cords substantially parallel to one another, such as for instance a belt strip or other semi-finished reinforcing products intended for manufacturing the carcass of a green tire for vehicles.
In the following description and in the subsequent claims, the term: “ribbon of rubber-coated fabric”, is used to indicate any continuous strip of rubber mixture embedding a plurality of reinforcing cords, by way of example metal and/or textile cords substantially parallel to one another.
The present invention also relates to an apparatus for cutting a ribbon of rubber-coated fabric including a plurality of reinforcing cords as defined above.
2. Description of the Related Art
As is known, a green tire is essentially constituted by a composite structure obtained by assembling together a plurality of semi-finished products and comprising: an airproof liner; at least one reinforcing ply; two bead cores to which the ply edges are associated; an elastomeric filling fixed to the radially outer surface of each bead core; a belt structure arranged as a crown around the torically shaped carcass ply generally comprising at least one pair of radially superposed belt strips provided with reinforcing cords crossed with each other and inclined with respect to the circumferential direction of the tire; a tread band wrapped around the belt structure; and a pair of axially opposite sidewalls radially extending between the bead cores and the tread band. The belt structure may also comprise a further layer generally arranged at the radially outermost position, incorporating a plurality of circumferentially oriented reinforcing cords, otherwise known as zero degree cords.
In order to manufacture semi-finished reinforcing products, such as for example the belt strips intended for forming the above-mentioned belt structure, a ribbon of rubber-coated fabric is generally utilized, said ribbon including a plurality of reinforcing cords substantially parallel to one another and inclined of a predetermined angle with respect to a central longitudinal plane of the ribbon (said angle being typically comprised between about 15° and 30°), which ribbon is cut to size according to a cutting direction as parallel as possible to the direction of the reinforcing cords in order to minimize the risk of cutting or damaging one or more cords. The damaged reinforcing cords, in fact, and particularly those emerging from the ribbon in which they are incorporated (know in the art with the term of “wild wires”), cause not only the generation of a semi-finished product of unacceptable quality, but may also cause the jamming of the automatic machines utilized for manufacturing the green tires, with a consequent need of labor intervention and an increase in production costs. Besides, if an imprecise cutting of the ribbon of rubber-coated fabric should cause a complete breaking of the reinforcing cords (such as for example the breaking of one or more threadlike elements of the cord and/or one or more strands of the cord) or a divarication of the free end of a cord (known in the art with the term of “splayed point cord”), the further risk of a defect difficult to be detected by means of a conventional quality control of the tire arises, so that the defect can become visible only in operation, leading even to a precocious and dangerous damages of the tire structure.
In view of above, one of the objects of the art of the field is that of preventing an imperfect cutting of the ribbon of rubber-coated fabric from negatively affecting both the tire quality and the productivity of the plants for manufacturing green tires.
To this purpose, for example, cutting apparatuses provided essentially with a blade intended, in a first step, for penetrating the rubber between two adjacent cords, approximately in the middle of the ribbon of rubber-coated fabric, are used, forming in this manner a through-notch of predetermined length and, in a second step, for cutting the ribbon of rubber-coated fabric from one side to the other, starting from the above-mentioned notch. In some cases, said steps of penetration (i.e. of incision) within the rubber of the fabric and of cutting are carried out, respectively, by two blades instead of one.
For instance, patents U.S. Pat. No. 2,665,757 and U.S. Pat. No. 3,641,855 describe a cutting apparatus essentially constituted by a pair of identical blades movable along a direction parallel to the cords, each one of which provided with a sharpened portion adapted to penetrate the ribbon of fabric, and provided with a blunt portion to tear the rubber of the ribbon present between adjacent cords. On the basis of the cutting methods of the ribbon of rubber-coated fabric illustrated therein, the blades, by means of their sharpened portion, make a starting notch in the ribbon and, subsequently, by moving in opposite directions starting from the above-mentioned starting notch, complete said cutting while theoretically remaining between two adjacent cords.
The cutting methods and apparatuses described in the above documents, however, imply the non negligible risk of damaging or cutting the reinforcing cords, generating thereby the so-called “wild wires” during the starting incision step of the ribbon of rubber-coated fabric by the sharpened portions of the blades.
Besides, the cutting apparatus described in document U.S. Pat. No. 2,665,757 comprises two presser feet intended for holding in a fixed position the ribbon portion positioned astride the cutting zone, in order to facilitate the cutting operations.
However, in this manner, because of the rigid constraint to which said ribbon is subjected during the cutting operation, it has been found that the cords may be passed over and irreversibly damaged by the blade when the cords have—because of stretches or strains undergone by the ribbon during the preceding working and handling steps to which the ribbon is submitted—a different inclination with respect to the nominal inclination, on the basis of which the cutting direction is set.
A further similar cutting apparatus, showing the same drawbacks illustrated above, is described in patent U.S. Pat. No. 4,608,890. Said apparatus comprises only one blade performing a starting notch of the ribbon of rubber-coated fabric and completing the cut starting from said starting notch by means of a stroke extending between the opposite lateral edges of the ribbon.
Further cutting methods and apparatuses, described for instance in patents U.S. Pat. No. 4,156,378 and U.S. Pat. No. 4,210,042, employ a circular blunt blade or a pair of circular blunt blades able to engage, by a rotating motion, the ribbon of rubber-coated fabric and to cut the same between a pair of adjacent cords. Also in this case, it has been found that the cords may be easily passed over and irreversibly damaged by the circular blade rotating and translating when the cords have a different inclination with respect to the nominal inclination.
The above-mentioned drawbacks, not negligible even per se, are further amplified both when the cords have a reduced diameter (by way of example smaller than about 0.4-0.7 mm) and as the width of the ribbon of rubber-coated fabric to be submitted to the cutting operation increases (by way of example greater than about 350 mm) because of the following reasons.
Firstly, a trend to progressively reduce the diameter of the reinforcing cords has been spreading, which trend is mainly dictated by the attempt of reducing the moment of inertia of the tire and the overall weight of the same. Said trend increases the risk of cutting or damaging the cords, if markedly pointed or sharpened portions of the blades are utilized, as stressed above with reference to patents U.S. Pat. No 2,865,757, U.S. Pat. No. 3,641,855 and U.S. Pat. No. 4,608,890. At the same time, said trend necessarily implies the requirement of increasing the density of the reinforcing cords in order to ensure the same mechanical strength which would be obtained by using cords having greater diameters and, with this requirement, the increased risk of damaging or even cutting a cord during the starting incision step of the ribbon of rubber-coated fabric.
Secondly, the trend of employing tires having a footprint area always increasing, necessarily results in an increase of the width of the semi-finished products of rubber-coated fabric such as, for example, the belt strips. Said increase of the width necessarily causes the increase of the cutting path and, therefore, the likelihood of impairing the quality of the semi-finished product due to a cut made in an incorrect manner.
In addition to this trend and associated thereto, in order to increase the performance of the tire, the necessity of reducing the inclination angle of the reinforcing cords with respect to the central longitudinal axis has arisen. Hence, the necessity of performing a precise cut in terms of holding a cutting direction as constant as possible throughout the whole cutting path of the ribbon of rubber-coated fabric.
On the other hand, even if it were possible to ensure a substantially constant cutting direction throughout the whole cutting path of the ribbon of rubber-coated fabric, the risk of passing over and damaging one or more cords persists because of the risk of stretches or strains of the fabric in the preceding working steps, which risk is particularly high during the storage on a reel of the ribbon of rubber-coated fabric or in any case during the other handling operations of the same. Said stretches or strains of the ribbon of rubber-coated fabric, which are responsible for the deviation of the inclination of the reinforcing cords with respect to the nominal inclination, contribute to the difficulty of cutting a ribbon of rubber-coated fabric in a precise and repeatable manner without damaging the reinforcing cords thereof.